U.S. patent application number 11/441305 was filed with the patent office on 2006-12-28 for combined mass and differential mobility spectrometry and associated methods, systems, and devices.
Invention is credited to Daren Levin, Raanan A. Miller, Erkinjon G. Nazarov, Paul Vouros.
Application Number | 20060289745 11/441305 |
Document ID | / |
Family ID | 36954346 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060289745 |
Kind Code |
A1 |
Miller; Raanan A. ; et
al. |
December 28, 2006 |
Combined mass and differential mobility spectrometry and associated
methods, systems, and devices
Abstract
The invention relates generally to systems, methods and devices
for analyzing samples and, more particularly, to systems using a
mass analyzer in combination with a differential mobility
spectrometer to enhance the analysis process of constituents of a
sample.
Inventors: |
Miller; Raanan A.; (Chestnut
Hill, MA) ; Nazarov; Erkinjon G.; (Lexington, MA)
; Vouros; Paul; (Concord, MA) ; Levin; Daren;
(Apex, NC) |
Correspondence
Address: |
FISH & NEAVE IP GROUP;ROPES & GRAY LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110-2624
US
|
Family ID: |
36954346 |
Appl. No.: |
11/441305 |
Filed: |
May 24, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60684411 |
May 24, 2005 |
|
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60713392 |
Sep 1, 2005 |
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Current U.S.
Class: |
250/294 |
Current CPC
Class: |
H01J 49/004 20130101;
H01J 49/0031 20130101; G01N 27/624 20130101 |
Class at
Publication: |
250/294 |
International
Class: |
H01J 49/28 20060101
H01J049/28 |
Claims
1. A method for analyzing a sample comprising: generating a first
spectra based on the ion mobility of a first portion of ions of the
sample, and generating a second spectra based on the mass-to-charge
ratio of a second portion of ions of the sample.
2. The method of claim 1 comprising generating a first standard
spectrum based on the ion mobility of at least one known ion
species.
3. The method of claim 2 comprising generating a second standard
spectra based on the mass-to-charge ratio of at least one known ion
species.
4. The method of claim 2 comprising storing a set of conditions
associated with the at least one ion intensity peak of the first
standard spectra.
5. The method of claim 4, wherein the set of conditions include at
least one of a time-varying voltage and a compensation voltage for
at least one ion mobility based filter.
6. The method of claim 5, wherein the ion mobility based filter
includes at least one of a differential mobility spectrometer and
an ion mobility spectrometer.
7. The method of claim 4, wherein the set of conditions includes at
least one of a type of dopant, a concentration of a dopant,
pressure, temperature, flow rate, and mass analyzer voltages
associated with generating the first and second spectra.
8. The method of claim 2 comprising comparing the first spectra to
the first standard spectra to identify at least one ion
species.
9. The method of claim 3 comprising comparing the second spectra to
the second standard spectra to identify at least one ion
species.
10. The method of claim I comprising filtering the second portion
of ions of the sample based on ion mobility and generating the
second spectra from the filtered second portion of ions.
11. The method of claim 10 comprising integrating the area of at
least one ion intensity peak of the second spectra associated with
at least one ion species to quantize the at least one ion species
in the sample.
12. The method of claim 11 comprising matching at least one ion
intensity peak of the second spectra with a known standard spectra
of a known ion species and adjusting the ion species quantization
based on a predicted deviation of the quantization of the known ion
species under similar conditions.
13. The method of claim 12, wherein the predicted deviation is
based at least in part on molecular modeling.
14. The method of claim 13, wherein the molecular modeling include
at least one of a Core and Facade mechanism.
15. The method of claim 1 comprising directly infusing the sample
using nano-electrospray ionization.
16. A sample analysis system comprising, at least one ion mobility
based analyzer including: an ion mobility based filter for
generating a time-varying electric field and compensation field
through which ions of the sample flow along a flow path, an ion
mobility based detector for detecting a first portion of the ions
in the flow path, at least one mass analyzer for detecting a second
portion of the ions delivered from the at least one ion mobility
based analyzer, and a controller for generating a first spectra
associated with the first portion of ions and generating a second
spectra associated with the second portion of ions.
17. The system of claim 16, wherein the controller is configured
for generating a first standard spectrum based on the ion mobility
of at least one known ion species.
18. The system of claim 17, wherein the controller is configured
for generating a second standard spectra based on the
mass-to-charge ratio of at least one known ion species.
19. The system of claim 17 comprising a data store for storing a
set of conditions associated with the at least one ion intensity
peak of the first standard spectra.
20. The system of claim 19, wherein the set of conditions include
at least one of a time-varying voltage and a compensation voltage
for at least one ion mobility based filter.
21. The system of claim 16, wherein the ion mobility based filter
includes at least one of a differential mobility spectrometer and
an ion mobility spectrometer.
22. The system of claim 19, wherein the set of conditions includes
at least one of a type of dopant, a concentration of a dopant,
pressure, temperature, flow rate, and mass analyzer voltages
associated with generating the first and second spectra.
23. The system of claim 17, wherein the controller is configured
for comparing the first spectra to the first standard spectra to
identify at least one ion species.
24. The system of claim 18 wherein the controller is configured for
comparing the second spectra to the second standard spectra to
identify at least one ion species.
25. The system of claim 16, wherein the controller is configured
for integrating the area of at least one ion intensity peak of the
second spectra associated with at least one ion species to quantize
the at least one ion species in the sample.
26. The system of claim 25, wherein the controller is configured
for matching at least one ion intensity peak of the second spectra
with a known standard spectra of a known ion species and adjusting
the ion species quantization based on a predicted deviation of the
quantization of the known ion species under similar conditions.
27. The system of claim 26, wherein the predicted deviation is
based at least in part on molecular modeling.
28. The method of claim 27, wherein the molecular modeling include
at least one of a Core and Facade mechanism.
29. The system of claim 16 comprising at least one electrospray
ionization source.
30. The system of claim 29, wherein the electrospray ionization
source includes a direct infusion ionization source.
31. The system of claim 16, wherein the ion mobility based analyzer
and mass analyzer are micromachined and included in an integrated
circuit package.
32. The system of claim 29, wherein the ion mobility based
analyzer, mass analyzer, and electrospray ionization source are
included in an integrated circuit package.
33. The system of claim 29, wherein the ion mobility based analyzer
and electrospray ionization source are included in an integrated
circuit package.
34. The system of claim 33 comprising an interface for detachably
connecting the integrated package to the mass analyzer.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of: U.S.
Provisional Application No. 60/684,411, filed on May 24, 2005,
entitled "Methods and Systems To Characterize Differential Mobility
Spectrometry (DMS) Gas Phase Molecular Interaction Mechanisms and
Their Use in Predicting Differential Mobility Ion Behavior" and
U.S. Provisional Application No. 60/713,392, filed on Sep. 1, 2005,
entitled "DMS Compensation Voltage Scanning of a Selected Mass
Spectrometer Mass-to-Charge Ratio Ion Signal For Rapid Analyte
Quantization From a Directly Infused ESI Sample," all of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The invention relates generally to systems, methods and
devices for analyzing samples. More particularly, in various
embodiments, the invention relates to systems and related methods
using mass spectrometry in combination with differential mobility
spectrometry to enhance the analysis process of constituents of a
sample.
BACKGROUND
[0003] There are a number of different circumstances in which it is
desirable to perform an analysis to identify and/or measure
compounds in a sample. Such samples may be taken directly from the
environment or they may be provided by front end specialized
devices to separate or prepare compounds before analysis.
[0004] Differential Mobility Spectrometry (DMS), also referred to
as High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS)
and Field Ion Spectrometry (FIS), are technologies for gas phase
ion sample separation and analysis. Researchers have interfaced DMS
with mass spectrometry to take advantage of the atmospheric
pressure, gas phase, and continuous ion separation capabilities of
DMS and the detection specificity offered by mass spectrometry.
[0005] By interfacing DMS with mass spectrometry, researchers have
demonstrated benefits in numerous areas of sample analysis,
including proteomics, peptide/protein conformation,
pharmacokinetic, and metabolism analysis. In addition to
pharmaceutical/biotech applications, DMS has been incorporated into
products designed for trace level explosives detection as well as
petroleum monitoring. Despite the demonstrated success of the
technology, ion behavior during the differential mobility
separation is not well understood for the wide range of analytes
that are being analyzed by this technology. Accordingly, there is a
need to enhance separation efficiency and enable predictable
separation controls for a differential mobility spectrometer (DMS)
interfacing with a mass spectrometer (MS).
[0006] DMS can be viewed as a spin off from conventional ion
mobility spectrometry (IMS). In conventional IMS, ions are pulsed
into and then pulled through a flight tube by a constant electric
field. The ions interact with a bath gas in the flight tube and the
interactions affect the time it takes an ion to pass through the
flight tube. Unlike time of flight (TOF) MS where an ion's time
through a flight tube is based solely on its mass to charge ratio
(due to collision free travel in a vacuum), ions in conventional
IMS are not separated in a vacuum, enabling interactions with the
bath gas molecules. These interactions are specific for each
analyte ion of a sample, leading to an ion separation based on more
than just mass/charge ratio.
[0007] DMS is similar to conventional IMS in that the ions are
separated in a bath or drift gas. However, typically, an asymmetric
electric field waveform is applied to two parallel electrode plates
through which the ions pass in a continuous, non-pulsed, manner.
The electrical waveform consists of a high field duration of one
polarity and then a low field duration of opposite polarity. The
duration of the high field and weak field portions are applied such
that the net voltage being applied to the electrode is zero. FIG. 1
includes an illustration 100 of the high and low voltages of
opposite polarity applied to generate the asymmetric electrical
waveform (identified as an Rf (field) voltage, correlating to the
high voltage value) and a conceptual diagram of a DMS filter 102
where the path of an ion M.sup.+ is subject to an asymmetric field
resulting from the asymmetric waveform. As can be seen in
illustration 100, after one cycle of the waveform, the net voltage
applied to the DMS filter electrode is zero.
[0008] However, the ion's mobility in this asymmetric electric
field demonstrates a net movement towards the bottom electrode
plate of the DMS filter 102. This example shows that, in DMS, an
ion's mobility is not proportional under the influence of a low
electric field compared to a high electric field. Since an ion may
experience a net mobility towards one of the electrode plates
during its travel between the plates, a compensation voltage (Vc)
is applied to maintain a safe trajectory for the ion through the
DMS filter 102 plates without striking them. The ions are passed
between the two electrodes by either being pushed through with a
pressurized gas flow upstream of the electrode plates or pulled
through by a pump downstream from the electrodes.
[0009] In conventional IMS, as well as DMS, ions are separated in a
gas at pressures sufficient for the occurrence of collisions
between ions and the neutral gas molecules. The smaller the ion,
the fewer collisions it will experience as it is pulled through the
drift gas. Because of this, an ion's cross sectional area may play
a significant role in it's mobility through the drift gas. As shown
in FIG. 1, an ion's mobility is not proportional under the
influence of a low electric field compared to a high electric
field. This difference in mobility may be related to
clustering/de-clustering reactions taking place as an ion
experiences the weak and strong electric fields. An ion experiences
clustering with neutral molecules in the drift gas during the weak
field portion of the waveform, resulting in an increased cross
sectional area. During the strong field portion of the waveform,
the cluster may be dissociated, reducing the ion's cross sectional
area.
[0010] Recently, DMS research has focused on understanding the gas
phase molecular interactions taking place and how they influence an
ions' mobility in the DMS sensor. Existing FAIMS-MS systems have
demonstrated DMS separation between certain monomer ions and
non-covalently bound cluster/dimer ions. These systems have
provided evidence that non-covalently bound dimer/cluster ions can
have different differential mobility behavior from their monomer
counterparts. This indicates that the cluster/dimer ions were
created prior to entering the DMS sensor, and were not dissociated
back to their monomer counterparts upon entering the asymmetric
electric field of the sensor and/or DMS filter. The formation of
these cluster/dimer ions may effect the detection of ions of
interest within a sample analysis system such as a DMS-MS.
Accordingly, there is a need for compensating for and/or accounting
for the presence of cluster/dimer ions and other compounds that
result from gas phase molecular interactions in a sample analysis
system to enhance the accuracy and resolution of these systems.
[0011] Existing DMS-based systems have analyzed sample ions through
the use of various vapor modified drift gases for which the
proposed process is via clustering/de-clustering interactions
between a monomer analyte ion and neutral drift gas modifier/dopant
molecule in which the analyte ion's effective cross sectional area
is changed. While existing DMS-based systems have shown a change in
an analyte ion's differential mobility behavior through the use of
drift gas modifiers or dopants, there remains a need for a clear
model with regards to the underlying interactions between the
modifier and analyte, and the mechanism(s) by which those
interactions change an analyte ion's differential mobility
behavior.
[0012] By employing a DMS as a pre-filter to a MS, existing
FAIMS-MS and/or DMS-MS systems have increased the detection
sensitivity and resolution of sample analysis by reducing the
amount of contaminants or unwanted particles that interact with the
ions of interest in a MS. Electrospray ionization (ESI) has been
employed with FAIMS-MS to facilitate the analysis of certain liquid
samples. However, direct infusion of samples using ESI has
typically been avoided, particularly with complex samples, because
of problems with competitive ion suppression. Competitive ion
suppression has limited the accuracy of existing ESI-FAIMS-MS
systems by reducing the quantity of ions of interest that are
eventually detected in the MS. Because of ion suppression, analyte
separation techniques prior to ESI, such as Liquid Chromatography
(LC), Gas Chromatography (GC), and Capillary Electrophoresis (CE),
have been utilized to minimize ion suppression effects.
Accordingly, there is a need for providing an ESI-DMS-MS system
having enhanced capabilities that reduce competitive ion
suppression and/or compensate for the effects of such suppression
when quantizing certain ion species.
SUMMARY
[0013] The invention, in various embodiments, addresses
deficiencies in the prior art by providing systems, methods and
devices for detecting, identifying, measuring and/or analyzing
(collectively "analyzing") constituents in a sample. More
particularly, the invention provides enhanced control, modeling,
and analysis techniques to improve the detection and quantization
of constituents in a sample. The samples and constituents may
include any material; chemical or biological, organic or
inorganic.
[0014] In certain illustrative embodiments, the invention is
directed to an ESI-DMS-MS combination system, which employs
enhanced modeling techniques to compensate for effects of
competitive ion suppression and, thereby, provide better sample
resolution and quantization. The ESI-DMS-MS system may also employ
enhanced predictive separation control based on predictive modeling
of various gas phase molecular interactions for certain analytes of
interest. The ESI-DSM-MS system may employ molecular modeling to
predict the influence of drift gas modifications on analyte ion
separation and, thereby, interpolate or estimate the actual
quantity of a particular analyte within a sample more accurately.
Such predictive modeling, in certain embodiments, may be applied to
analyzing samples including constituent identification. The
molecular modeling may enable the altering or controlling of an
analyte ion's differential mobility behavior based on gas phase
molecular clustering interactions.
[0015] The molecular model, in one feature, accounts for the
influence of chemical structure, conformational freedom, H-bonding,
electrostatic attraction, and steric repulsion on gas phase
interactions and the mechanisms by which they alter an analyte
ion's differential mobility behavior. More particularly, two gas
phase interaction mechanisms, e.g., the Core and Facade mechanisms,
are employed which detail drift gas modifier effects on analyte ion
differential behavior.
[0016] According to one aspect, the invention provides a sample
analysis system having at least one ion mobility based analyzer.
The ion mobility based analyzer includes an ion mobility based
filter for generating a time-varying electric field and
compensation field through which ions of the sample flow along a
flow path. The ion mobility based analyzer also includes an ion
mobility based detector for detecting a first portion of the ions
in the flow path. The sample analysis system also includes at least
one mass analyzer for detecting a second portion of the ions
delivered from the at least one ion mobility based analyzer. The
sample analysis system further includes a controller for generating
a first spectra associated with the first portion of ions and
generating a second spectra associated with the second portion of
ions.
[0017] In one feature, the controller is configured for generating
a first standard spectrum based on the ion mobility of at least one
known ion species. In another feature, the controller is configured
for generating a second standard spectra based on the
mass-to-charge ratio of at least one known ion species. In one
configuration, a data store stores a set of conditions associated
with at least one ion intensity peak of the standard spectra.
[0018] The set of conditions may include a time-varying voltage or
a compensation voltage for at least one ion mobility based filter.
At least one ion mobility based filter may be a differential
mobility spectrometer or an ion mobility spectrometer. The set of
conditions may also include, without limitation, a type of dopant,
a concentration of a dopant, pressure, temperature, flow rate, or
mass analyzer voltages associated with generating the first and
second spectra.
[0019] In one configuration, the controller is configured for
comparing the first spectra to the first standard spectra to
identify at least one ion species. In another configuration, the
controller is configured for comparing the second spectra to the
second standard spectra to identify at least one ion species.
[0020] In a further configuration, the controller is configured for
integrating the area of at least one ion intensity peak of the
second spectra associated with at least one ion species to quantize
at least one ion species in the sample. The controller may then
match at least one ion intensity peak of the second spectra with a
known standard spectra of a known ion species and adjust the ion
species quantization based on a predicted deviation of the
quantization of the known ion species under similar conditions. The
predicted deviation may be based, at least in part, on certain
molecular modeling. The molecular modeling may include at least one
of a Core and Facade mechanism or a combination of both.
[0021] In one feature, the sample analysis system inlcudes at least
one electrospray ionization source. The electrospray ionization
source may include a direct infusion ionization source. In another
feature, the ion mobility based analyzer and mass analyzer are
micromachined and included in an integrated circuit package. In a
further feature, the ion mobility based analyzer, mass analyzer,
and electrospray ionization source are included in an integrated
circuit package. In yet another feature, the ion mobility based
analyzer and electrospray ionization source are included in an
integrated circuit package. In one configuration, the sample
analysis system includes an interface for detachably connecting the
integrated package to the mass analyzer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] These and other features and advantages of the invention
will be more fully understood by the following illustrative
description with reference to the appended drawings, in which like
elements are labeled with like reference designations and which may
not be to scale.
[0023] FIG. 1 includes an illustration of the high and low voltages
of opposite polarity applied to generate the asymmetric electrical
waveform and a conceptual diagram of a DMS filter where the path of
an ion M.sup.+ is subject to an asymmetric field resulting from the
asymmetric waveform.
[0024] FIG. 2 is a conceptual diagram of a ESI-DMS-MS system
according to an illustrative embodiment of the invention.
[0025] FIG. 3 is a flow diagram of an exemplary sample analysis
method for accurate ion identification and quantization according
to an illustrative embodiment of the invention.
[0026] FIG. 4 is a combined graphic display of a three-dimensional
plot and an associated two-dimensional plot at Rf=900v and
associated mass spectrometric scans at Vc=-5.3 volts and Vc=-2.5
volts according to an illustrative embodiment of the invention.
[0027] FIG. 5 is a combined graphic view of a multi-scan plot, an
associated calibration table of peaks of the multi-scan plot, and
calibration curve for 25 ug/ml Ang. Frag. according to an
illustrative embodiment of the invention.
[0028] FIG. 6 is a combined graphic view of a mass spectrometric
spectra and associated calibration tables for 2 ug/ml and 10 ug/ml
Ang. spiked into a mixture of seven peptides according to an
illustrative embodiment of the invention.
[0029] FIG. 7 is a combined graphic view of a three-dimensional
plot and an associated two-dimensional plot at Rf=1000v and
associated mass spectrometric spectra at Vc=-9 volts and Vc=-10.25
volts respectively according to an illustrative embodiment of the
invention.
[0030] FIG. 8 is a bar graph of the percentage of dimer ion signal
to monomer ion signal for each analyte with no drift gas modifier
and a DMS pre-filter turned off.
[0031] FIG. 9 is a combined graphic view the DMS spectra for
3-hydroxy piperidine with no gas modifier and associated mass
spectrometric spectra at Vc=-5.5 volts (monomer Vc point) and
Vc=-1.25 volts (dimer Vc point) respectively.
[0032] FIG. 10A is an exemplary view of a dimer ion structure that
may be present in the monomer Vc point of FIG. 9.
[0033] FIG. 10B is an exemplary view of a dimer ion structure that
is formed as a shared proton between two neutral analyte molecules
which may be present in the dimer Vc point of FIG. 9.
[0034] FIG. 11 is an exemplary view of a protonated piperidine and
neutral 2-propanol molecule.
[0035] FIG. 12 is a combined graphic view of the DMS spectra for
piperidine at an Rf=1000 volts with no modifier and associated mass
spectrometric spectra and the DMS spectra with a 2-propanol
modifier and associated mass spectrometric spectra.
[0036] FIG. 13 is a combined graphic view of the DMS spectra for
dymethyl-piperidine at an Rf=1000 volts with no modifier and
associated mass spectrometric spectra and the DMS spectra with a
2-butanol modifier and associated mass spectrometric spectra.
[0037] FIG. 14 is a combined graphic view of 3 hydroxy-piperdine at
an Rf=1000 volts showing the DMS spectra with no modifier, a
2-propanol modifier, and a 2-butanol modifier along with their
associated mass spectrometric spectra respectively.
[0038] FIG. 15 is a combined graphic view of pentamethyl-piperdine
at an Rf=1000 volts showing the DMS spectra with no modifier, a
cyclopentanol modifier, a 2-butanol modifier, and a 2-propanol
modifier along with their associated mass spectrometric spectra
respectively.
[0039] FIG. 16A is an exemplary view of a protonated
pentamethyl-piperidine complex with and without an associated
electron density cloud.
[0040] FIG. 16B is an exemplary view of a neutral 2-propanol
complex with and without an associated electron density cloud.
[0041] FIG. 17 is an exemplary view of the charge in conformational
freedom visual representation for tetramethyl-piperidine monomer
ion including the minimum energy conformation and superimposed
eight lowest energy conformations.
[0042] FIG. 18 is a combined graphic view of various monomer ion
equilibrium plots of Rf versus Vc under various conditions.
[0043] FIG. 19 is a combined graphic view of the DMS spectra for
piperidine dimer at an Rf=1000 volts with no modifier and
associated mass spectrometric spectra and the DMS spectra with a
2-propanol modifier and associated mass spectrometric spectra.
[0044] FIG. 20 is a combined graphic view of the DMS spectra for
Dimethyl-piperidine dimer at an Rf=1000 volts with no modifier and
associated mass spectrometric spectra and the DMS spectra with a
2-butanol modifier and associated mass spectrometric spectra.
[0045] FIG. 21A is a Rf versus Vc plot of the piperidine dimer ion
equilibrium under various conditions.
[0046] FIG. 21B is a Rf versus Vc plot of the
Cis-dimethylpiperidine dimer ion equilibrium under various
conditions.
[0047] FIG. 21C is a Rf versus Vc plot of the 3-Hydroy piperidine
dimer ion equilibrium under various conditions.
ILLUSTRATIVE DESCRIPTION
[0048] The invention, in various embodiments, provides systems,
methods and devices for detecting, identifying, measuring and
analyzing (collectively "analyzing") constituents in a sample. The
samples and constituents may include any material; chemical or
biological, organic or inorganic. In particular illustrative
embodiments, the invention is directed to an ESI-DMS-MS combination
system, which employs enhanced modeling techniques to compensate
for effects of competitive ion suppression and, thereby, provide
better sample resolution and quantization.
[0049] In one embodiment, Electrospray ionization (ESI) combined
with Differential Mobility Spectrometry (DMS) and Mass Spectrometry
(MS) is utilized for rapid analyte quantization of a directly
infused ESI sample. In another embodiment, the ESI-DMS-MS system
includes a micromachined, nanomachined, and/or nanoESI-DMS-MS
platform for rapid quantitative analysis.
[0050] FIG. 2 is a conceptual diagram of a ESI-DMS-MS system 200
according to an illustrative embodiment of the invention. The
ESI-DMS-MS system 200 includes a nanospray source 202, a flow path
208, a DMS filter 210, a DMS detector 212, a MS analyzer 240, an MS
inlet cone 228, a drift gas inlet 226, a mixing region 222, a vial
244, a dopant reservoir 224, a DMS analyzer inlet 206, an ESI inlet
204, a DMS analyzer housing 236, DMS filter electrodes 214 and 216,
DMS ion detector plates 218 and 220, substrates 232 and 234,
electronic controller 242, and orifice 238. The DMS filter 210 may
include a Sionex SDP-1 modified sensor, while the MS analyzer 240
may include a Micromass ZQ detector. The electronic controller 242
may include a microprocessor for regulating one or more conditions
of the ESI source 202, the flow path 208, the DMS filter 210, the
DMS detector 212, the mixing region 222, and the MS analyzer
240.
[0051] For example, the controller 242 may adjust the compensation
voltage (Vc) applied to at least one of the filter electrodes 214
and 216 or a condition of a time-varying voltage waveform (Vrf) to
the filter electrodes 214 and 216 to effect ion separation in the
DMS filter 210. A condition of the filter 210 may include the
asymmetry, duty cycle, magnitude, frequency of the waveform (See
FIG. 1). The controller 242 may regulate the flow of N.sub.2 gas
through the inlet 226, the mixing period and/or amount within
region 222, and the amount and/or flow rate of mixed or unmixed gas
that is introduced via inlet 206 into the flow path 208. The
controller 242 may control the operation of the ESI source 202. The
controller 242 may further control the operation of the MS analyzer
240. The controller 242 may interface with a data store 244 which
may include a database, list, array, or like data structure
containing information, such as condition information, associated
with system 200.
[0052] In one embodiment, the ESI-DMS-MS system 200 is contained
within a single integrated circuit (IC) package. In another
embodiment, the ESI-DMS is included in an IC package that includes
an interface portion capable of detachable connection to a standard
MS analyzer 240.
[0053] In operation, a sample S is introduced into the flow path
208 via the ESI inlet 204 from the ESI source 202. The sample S may
originally be in a liquid form until processed at the ESI source
202 and injected into the flow path 208 as a spray of ions. The
ions are then transported by a drift and/or carrier gas, introduced
via inlet 206, to the DMS filter 210. While passing through the DMS
filter 210, the ions are subject to a time-varying electric field
and compensation field that separates and/or allows certain ion
species to pass through the filter 210 while other ions are
directed toward one of the filter electrodes 214 or 216 and
neutralized. A portion of the ions that reach the detector 212 may
be detected by one or both of the detector electrodes 218 and 220.
In one embodiment, certain ions are directed through the orifice
238, embedded in electrode 220 to the MS analyzer 240 via the inlet
cone 228 for mass spectrometric detection. The controller 242 may
process either or both of the detected spectra from the DMS
detector 212 and the MS analyzer 240. The controller 242 may
include a processor for executing software, firmware, and/or
hardware programs that control a portion of the components and/or
operations of the system 200.
[0054] The carrier gas may include pure nitrogen, some other gas,
or a gas including one or more dopants. The controller 242 may
selectively adjust the dopant concentration within the carrier gas
depending on the ion species of interest and/or to be detected. The
controller 242 may include a data store, database, memory storage,
grouping, and/or list of condition information associated with
known ion species. The condition information, for example, may
include compensation voltage settings and/or peak ranges of the DMS
filter 210 associated with at least ion species and related MS
analyzer 240 spectra to enable identification of certain ion
species.
[0055] In one embodiment, the combined fast compensation voltage
(Vc) scanning capability of the DMS filter 210 (e.g., in
milliseconds) and the selective mass-to-charge (m/z) ion signal
generation of the MS analyzer 240, enable the rapid generation of a
selected ion DMS spectra with which the DMS peak area of the
selected m/z value is integrated and the peak apex compensation
voltage Vc is used for accurate analyte identification, similar to
how retention time is used for liquid or gas chromatography. Since
ESI can potentially produce isobaric background ions of the same
m/z as an analyte of interest, the use of the DMS peak apex
compensation voltage for matching with that of a reference standard
enables accurate identification of an analyte of interest from that
of a background ion signal which is particularly important at trace
quantization levels.
[0056] This approach to analyte analysis enables rapid quantization
of multiple analytes of interest from a single sample. The limiting
factor to the speed of analysis time is the scanning speed of the
MS analyzer 240, which needs to be fast enough to enable the
collection of sufficient data points across the DMS filter 210 Vc
spectra range. In one embodiment, the MS analyzer 240 includes of a
time-of-flight mass spectrometer having fast mass scanning
capabilities. Using the time-of-flight MS analyzer 240, it is
reasonable to generate a DMS spectra for each sample in the low
seconds (1-5 sec.) time frame for a large m/z window. The larger
the m/z window, the greater the number of selected ion DMS spectra
which are extracted from a single Vc scan, enabling the
quantization of multiple analytes from a single sample Vc scan (DMS
filter 210 spectra).
[0057] In certain embodiments, a slower scanning MS analyzers 240,
such as a quadrupole mass spectrometer, is employed. Thus, a single
ion monitoring mode may be required to maintain the mass scan
speeds necessary for the low second Vc scan times.
[0058] In other embodiments, the combined rapid Vc scanning of the
DMS filter 210, the selective m/z ion signal detection of MS
analyzer 240, the DMS filter 210 peak apex compensation voltage
matching, and the use of direct sample infusion to the ESI source
202, enable an ideal platform for ultra high-throughput analysis.
The use of a multi-sample, automated, direct infusion nanoESI
source 202, such as the Nanomate from Advion Biosciences Inc.,
combined with the DMS-MS separation, detection, and quantization
platform of system 200, encompasses the type of system capable of
performing ultra high-throughput analysis for numerous
applications. With regards to speed of analysis, the system 200
enables significantly faster sample analysis times compared to
other high-throughput approaches such as fast-LC, e.g., less than
about 5 seconds, less than about 4 seconds, less than about 3
seconds, less than about 2 seconds, and less than about 1
second.
[0059] While Flow Injection Analysis (FIA) and direct sample
infusion without the use of DMS are capable of achieving sample
analysis times, within an order of magnitude greater than 1-5
seconds, FIA and direct sample infusion do not provide any ion
separation prior to the MS analyzer 240. Thus, these systems cannot
provide the analyte specificity and quantitative accuracy expected
from the ESI-DMS-MS system 200. Both FIA and direct sample infusion
have been combined with the use of FAIMS-MS, however they have not
been used to rapidly generate selected ion DMS spectra via rapid Vc
scanning, and then utilize the DMS spectra peak area for
quantitative analysis. The present approach advantageously improves
sample analysis time, sensitivity, specificity, and quantitative
accuracy, compared to the other FAIMS-MS approaches. Preliminary
data indicates that the process of DMS Vc scanning may provide an
absolute increase in ion signal (via ion focusing or some other
process) compared to the mass spectra ion signal alone.
[0060] Direct infusion of samples with ESI has typically been
avoided, particularly with complex samples, because of problems
with competitive ion suppression. Because of ion suppression,
analyte separation techniques prior to ESI, such as Liquid
Chromatography (LC), Gas Chromatography (GC), and Capillary
Electrophoresis (CE), have been utilized to minimize ion
suppression effects. However, with the recent advances in low flow
nano-electrospray ionization, ion suppression can be minimized, and
possibly eliminated for many samples. In one embodiment, the
ESI-DMS-MS system 200 employs a nano-electrospray source 202 to
minimize ion suppression, enabling nanoESI-DMS-MS analysis platform
and/or system 200 to revolutionize sample analysis for a wide
spectrum of applications, such as quantitative and qualitative
analysis requiring faster throughput.
[0061] One of the primary analytical techniques being utilized in a
high-throughput manner for various applications is LC-MS. In
another embodiment, the nanoESI-DMS-MS analysis system 200 is
ideally suited to replace a LC-MS for many of the current
high-throughput applications, such as drug-discovery, ADME
(Adsorption, Distribution, Metabolism, Excretion),
biomarker/diagnostic screening, pharmacokinetic/pharmacodynamic,
and drug-protein binding. In addition, numerous drug product
quality control based assays for process control, product release,
and stability testing may benefit from the ESI-DMS-MS system 200.
Unlike LC based instruments, where each instrument can typically
only run a single method between column and mobile phase changes,
the nanoESI-DMS-MS analysis system 200 is capable of instantaneous
and/or concurrent automated switching between optimized DMS filter
210 settings for different analytes.
[0062] In a further embodiment, a slightly different approach to
operating the system 200, a specific MS analyzer 240 ion signal is
used to generate an ion specific DMS dispersion plot (Rf vs. Vc vs.
ion signal) instead of the DMS detector 212 spectra. In one
embodiment, an ion specific DMS dispersion plot may require more
analysis time to generate than the DMS spectra, but provides a
greater degree of analyte specificity due to analyte compensation
voltage matching across multiple time-varying Vrf voltages. The
generation of an ion specific DMS dispersion plots may be valuable
for numerous applications where an increased degree of analyte
specificity is desired.
[0063] FIG. 3 is a flow diagram 300 of an exemplary sample analysis
method for accurate ion identification and quantization according
to an illustrative embodiment of the invention. First, direct
sample S infusion into the nanoESI and/or ESI source 202 is
performed (Step 302). Next, all ions generated from an ESI plume
enter the DMS sensor and/or filter 210 via the flow path 208 for
ion separation (Step 304). A preliminary analyte specific
optimization of the DMS filter 210 conditions is performed with
reference to a standard whereby the peak apex compensation voltage
(Vc) is identified (Step 306). Then, the DMS filter 210 performs a
rapid compensation voltage scan using the optimized conditions
(e.g., compensation voltage range, Vrf, scan speed, and voltage
steps) (Step 308). The optimized condition settings of the DMS
filter 210 and other elements of the nanoESI-DMS-MS system 200 may
be stored in a data store and/or memory and retrieved at some later
time by the controller 242 to set the optimized nanoESI-DMS-MS
system 200 conditions. If optimization conditions are not available
and/or stored with the controller 242 data store, general
conditions know to work for certain ion species and/or classes may
be used (Step 310). Classes may include, without limitation, small
molecules, peptides, or proteins. Next, the system 200 uses an
analyte specific m/z signal from the MS analyzer 240 to create a
DMS spectra. During Vc scan, certain analyte ions will pass through
the DMS filter 210 and into the MS analyzer 240 over a specific Vc
range, creating a ion intensity peak (Step 312). Multiple m/z ion
signals may be extracted from a single analysis, DMS filter 210
scan, and/or DMS filter 210 ion intensity peak, to quantize
multiple analytes in a single sample (Step 314). Then, the
nanoESI-DMS-MS system 200 integrates the DMS ion intensity spectra
peak area and matches the peak apex compensation voltage against
the reference standard for accurate analyte identification and
quantization (Step 316).
[0064] For example, the nanoESI-DMS-MS system 200 may be employed
for peptide quantization to perform the above Vc scanning approach
with directly infused samples. The nanoESI-DMS-MS system 200 may
utilize control software, operated by the controller 242, that is
configured to identify the DMS peak apex compensation voltage for
certain rapid DMS filter 210 Vc scans and capable of averaging
multiple Vc scans to be displayed as one DMS spectra.
Peptide Quantization
[0065] In one experimental example, where the drift gas modifier
conditions were optimized, the use of the nanoESI-DMS-MS system 200
for rapid peptide quantization of directly infused samples was
investigated, utilizing the 8000 ppm 2-butanol drift gas modifier
condition for all analyses. The angiotensin (ang.) fragment peptide
was selected as the analyte of interest to be quantified. The
peptide samples for quantitation were prepared in
50/50/water/methanol with 0.1% formic acid for improved ionization
compared to the, 80/20 water/methanol with 0.1% formic acid, sample
solution conditions used for the optimization work.
[0066] In addition, the nanospray capillary tip position and mass
spectrometer and/or MS analyzer 240 conditions were optimized for
analyte ion sensitivity of the (M+H).sup.+ m/z 482 ion. The MS cone
228 voltage was increased to 40 V and the MS inlet source
temperature increased to 70.degree. C.
[0067] FIG. 4 is a combined graphic display 400 of a
three-dimensional plot 402 and an associated two-dimensional plot
404 at Rf=900v and associated mass spectrometric scans 406 and 408
at Vc=-5.3 volts and Vc=-2.5 volts respectively according to an
illustrative embodiment of the invention. The plot 402 is a
dispersion plot of Vrf versus Vc with ion intensity levels
indicated by varying shades of gray. In other embodiments, the plot
402 indicates ion intensity levels by varying color. In another
embodiment, the ion intensity may be shown as a surface of varying
elevations and/or contours. FIG. 4 shows the DMS dispersion plot
402, DMS spectra at Rf=900 V in plot 404, and the selected Vc point
mass spectra 406 and 408 at an Rf=900 V, for a 25 ug/ml ang.
fragment reference standard. As support that drift gas modifiers
facilitate the de-clustering of higher order peptide aggregate
ions, FIG. 4, as illustrated in plots 402, 404, 406, and 408,
demonstrates the differential mobility separation of the m/z 482
(M+H).sup.+ monomer ion at a Vc of -5.3 from the m/z 963
(2M+H).sup.+ dimer and 1444 (3M+H).sup.+ trimer ions at a lower Vc
of -2.5.
[0068] FIG. 5 is a combined graphic view 500 of a multi-scan plot
502, an associated calibration table 504 of peaks of the multi-scan
plot, and calibration curve 506 for 25 ug/ml Ang. Frag. according
to an illustrative embodiment of the invention. To create a
reference value reflective of 25 ug/ml angiotensin fragment, ten
(10) second Vc scans, at an Rf of 900 V, in m/z 482 selected ion
mode, scanning from -15 to 0 V, were collected and the peak area's
integrated. The plot 502 shows the m/z 482 peaks generated by six
repetitive Vc scans. Repeatability in the generation of the m/z 482
peaks through Vc scans was determined by calculating the percent
residual standard deviation (RSD) of the six replicate peaks areas.
A RSD of 4.6% for Vc scan peak area repeatability is shown in
calibration table 504. The average peak area value was used to
generate a calibration line and the equation y=116028.times. in
calibration curve 506, to be used for ang. fragment quantization
from a semi-complex peptide sample.
[0069] FIG. 6 is a combined graphic view 600 of a mass
spectrometric spectra 602 and associated calibration tables 604 and
606 for 2 ug/ml and 10 ug/ml ang. spiked into a mixture of seven
peptides according to an illustrative embodiment of the invention.
A sample was created which contained a mixture of the seven
different peptides, at 10 nmol/ml each, ranging in mass from 1045
to 1672 Da to create a semi-complex peptide mixture. Two ang.
fragment spiked recovery samples were prepared from this peptide
mixture, one with an ang. fragment concentration of 10 ug/ml and
the other 2 ug/ml. Six replicate 10 second Vc scans were generated
for each sample in the same manner as the 25 ug/ml reference
standard. FIG. 6 shows the mass spectra 602 for the 2 ug/ml spiked
recovery sample with the DMS sensor turned off, demonstrating the
ion complexity of the sample. The tables 604 and 606 show the
average m/z 482 peak area values and percent RSD for the 2 ug/ml
and 10 ug/ml samples respectively.
[0070] Based on the average peak area's, ang. fragment recovery
values of 90% and 91% for the 2 ug/ml and 10 ug/ml samples were
calculated from the equation of the calibration curve 506 of FIG. 5
respectively. The Vc scan % RSD and recovery data illustrate the
feasibility for rapid quantization of directly infused samples with
the nanoESI-DMS-MS system 200, utilizing fast Vc scanning. In
another emobiment, Ultra-rapid (0.5-5 sec. range) analysis times
are achievable with the fast Vc scanning capabilities of the DMS
filter 210 and a fast scanning MS analyzer 240, making the
quantitative analysis very desirable for numerous applications in
the high-throughput arena.
[0071] While certain analyzers have demonstrated a change in an
analyte ion's differential mobility behavior through the use of
drift gas modifiers and/or dopants, a clear model with regards to
the underlying interactions between the modifier and analyte, and
the mechanism(s) by which those interactions change an analyte
ion's differential mobility behavior, has not been developed.
Accordingly, in certain embodiments, the nanoESI-DMS-MS system 200
is configured and/or operated to account for the influence of
chemical structure, conformational freedom, H-bonding,
electrostatic attraction, and steric repulsion, on gas phase
interactions and the mechanisms by which they alter an analyte
ion's differential mobility behavior. The proposed mechanisms are
significant for a wide spectrum of DMS applications. Based on our
results, two gas phase interaction mechanisms which detail drift
gas modifier effects on analyte ion differential mobility behavior
are defined. Molecular modeling calculations with, for example,
CAChe software, enables an in-silico look at the proposed
mechanisms. While providing data with strong support of the
proposed mechanisms, the molecular modeling data also demonstrated
the potential for predictive determinations of differential
mobility behavior for an analyte with various drift gas modifiers.
In certain embodiments, these predictive determinations are used to
analyze a sample to identify certain sample constituents.
Illustrative Experimental Process And/Or System Instrumentation
[0072] The exemplary nanoESI-DMS-MS system 200, in one embodiment,
includes a small size of the DMS sensor (including the DMS filter
210 and DMS detector 212), being approximately 3'' in length, about
1'' in height, and about 1/4'' in width. The DMS sensor may also
include a simplified interface to the MS analyzer 240 inlet. In one
embodiment, the interface allows the DMS sensor to be detachably
connectable to the MS analyzer 240. In one configuration, samples
were infused into the nanospray ESI source 202 via a Harvard
syringe pump at a flow of about 1.25 .mu.L/min. Sample analysis was
performed in positive mode nanospray and a capillary voltage of
about 3.0 KV was applied. A cone voltage of about 12 V was applied
to the inlet MS cone 228 of the MS analyzer 240. The source
temperature for the MS analyzer 240 was set to about 40.degree.
C.
[0073] Returning to FIG. 2, in one embodiment, an ESI-DMS-MS system
200 includes a gas line via inlet 206 into the flow path 208 in the
DMS sensor (combined DMS filter 210 and DMS detector 212) opposite
of the nanospray inlet 204. This provides an introduction site for
the drift gas modifier vapors into the DMS sensor as well as a
curtain gas for the nanospray inlet 204. The two detector plates
218 and 220 immediately downstream from the separation electrodes
214 and 216 provide an ion signal for both positive and negative
ions. In one embodiment, a hole and/or orifice 238 is included in
one of the detector plates 220 to allow for ion transmission into
the MS analyzer 240. Despite the hole 238, in one embodiment, an
ion signal was still generated by the detector plate and/or
electrode 220. In another embodiment, the detector plates 218 and
220 are biased +5 and -5 volts depending on the ion signal polarity
desired. In certain embodiments, the positive ion detector plate
was assigned to the detector plate 220 with the hole 238. The
vacuum generated by the MS analyzer 240 provided the gas flow
(measured at approx. 1 L/min) through the DMS sensor and into the
MS analyzer 240. The gas line and inlet 226 opposite the DMS sensor
inlet 206 had a constant flow of nitrogen at approximately 0.7
L/min (with or without the modifier added). The drift gas modifiers
were introduced at a concentration of approximately 150 ppm of the
total gas flow through the DMS sensor. In certain embodiments, the
DMS sensor was operated at ambient environmental temperature.
Chemicals
[0074] Five related compounds, piperidine, cis-dimethyl-piperidine,
tetramethyl-piperidine, pentamethyl-piperidine, and
3-hydroxy-piperidine were used as test analytes in the ESI-DMS-MS
system 200. The samples were all prepared at 0.5mM in a solution of
90% HPLC grade water (sigma) and 10% HPLC grade methanol (sigma).
2-propanol, 2-butanol, and cyclopentanol were tested as the various
drift gas modifiers (sigma). As shown in FIG. 2, the drift gas
modifier vapors were introduced into the DMS sensor by filling a
5-mL glass reservior 224 with the appropriate modifier and placing
it within a gas trap style apparatus and/or vial 244, allowing the
modifier to mix with the drift gas in a mixing region 222 before
introduction into the DMS sensor. Each of the five analytes were
tested with and without all three drift gas modifiers.
Molecular Modeling
[0075] Molecular modeling experiments were performed with, for
example, CAChe Worksystem Pro Ver. 6.1.10 software (Fujitsu Corp.)
on a Compaq Presario 2100 laptop with an Athlon XP 1800+ processor
and 512 mb DDR RAM. Global minimum conformation energy values for
all complexes were determined by performing the following
experiment; property of: Chemical Sample Conformations (CAChe 5.0
experiments), property: Sequence of Conformations, Using: Global
Minimum search with MM2. Prior to performing the global minimum
conformation calculation for each complex, a single Chemical Sample
File containing each component molecule, within close proximity,
was created. The surface volume was determined for the minimum
energy conformation of the particular complex or individual ion.
The change in conformational freedom for a given analyte ion was
determined by superimposing the eight lowest energy conformations
for that analyte ion and calculating the surface volume of all
eight superimposed conformations. The difference in surface volume
between the minimum conformation versus all eight superimposed was
then determined.
Procedures
[0076] For each sample condition tested, a DMS dispersion plot was
generated from the DMS sensor positive ion detector plate 220
signal and by scanning compensation voltages (Vc) from about -20 to
+5 V for each Rf voltage ranging from about +500 to +1500 V, in
approximately 10 V increments.
[0077] FIG. 7 is a combined graphic view 700 of a three-dimensional
dispersion plot 702 and an associated two-dimensional plot 704 at
Rf=1000v and associated mass spectrometric spectra 706 and 708 at
Vc=-9 volts and Vc=-10.25 volts respectively according to an
illustrative embodiment of the invention. The sample differential
mobility dispersion plot 702 demonstrates DMS separation of various
ions. As the Vrf voltage is increased, the mobility/velocity of
some ions away from DMS filter electrode 214 and/or 216 is
increased, requiring a larger Vc of opposite polarity for safe
travel to the detector plate 220. A scan rate of about 1.03
sec/scan was used for each Vc scan, consisting of 100 steps between
about -20 and +5 volts, enabling an entire dispersion plot to be
generated in approximately 100 seconds.
[0078] From the dispersion plot 702 data, a DMS spectra (Vc vs. ion
signal), shown in plot 704, can be generated for a given Rf voltage
Vrf. For any Rf and Vc setting on the dispersion plot 702, a mass
spectra can be collected by the MS analyzer 240, providing insight
into the ion make up at a particular Vc point on the dispersion
plot 702. The point D and C are selected Vc points for which mass
spectra were collected, enabling the determination of the Vc
position corresponding to the maximum m/z 188 ion signal, shown in
plots 706 and 708. After the generation of a dispersion plot 702
for each sample condition, Rf and Vc points were selected
throughout the plot 702 to collect associated and/or matching mass
spectra via the MS analyzer 240. For each selected Rf and Vc
voltage setting, a mass spectra was collected which averaged about
30 seconds worth of 0.1 second mass scans. Mass spectra were also
collected at the MS analyzer 240 for each sample condition with the
DMS sensor turned off, allowing all the ions to enter the MS
analyzer 240. The combination of dispersion plots and the selected
Rf and Vc point mass spectra enabled the construction of accurate
Rf versus Vc plots of the maximum ion intensity for all the analyte
ions of interest. The Rf versus Vc plots identify the effects of
the drift gas modifiers on shifting the analyte ions' Vc for a
given Rf. Molecular modeling was used to examine the proposed
molecular gas phase interaction mechanisms taking place.
Dimer Ion Formation And Separation
[0079] In one experimental operation of the ESI-DMS-MS system 200,
five piperidine analytes were chosen to investigate chemical
differences in dimer/cluster ion formation. It was presumed that
the amine in each of the compounds would be the main site for dimer
formation. By sterically restricting access to the amine, through
methylating the adjacent C atoms, and the amine itself in the case
of pentamethyl-piperidine, a reduction in dimer formation was
expected. In contrast, with the addition of a hydrogen bonding
group, as in 3-hydroxy-piperidine, it was expected that dimer
formation would be enhanced.
[0080] FIG. 8 is a bar graph 800 that shows the percentage of dimer
ion intensity to monomer ion intensity, calculated from the mass
spectra collected by the MS analyzer 240, for each of the analyte
samples with the DMS sensor turned off. As illustrated, with the
increase in steric hindrance around the amine, dimer intensity
decreases. The pentamethyl-piperidine sample, having the most
limited access to the amine, demonstrated no detectable dimer ion.
In contrast, the 3-hydroxy-piperidine sample demonstrated a greater
than 2-fold increase in dimer signal compared to the piperidine
sample. For the analytes capable of generating sufficient dimer ion
signal, DMS separation using the DMS filter 210 between the monomer
and dimer ions was observed.
[0081] FIG. 9 is a combined graphic view 900 of the DMS spectra for
3-hydroxy piperidine with no gas modifier and associated mass
spectrometric spectra at Vc=-5.5 volts (monomer Vc point) and
Vc=-1.25 volts (dimer Vc point) respectively. FIG. 9 shows the DMS
spectra detected at the DMS detector 212 and corresponding mass
spectra detected by the MS analyzer 240 for the
3-hydroxy-piperidine sample with no drift gas modifier at an Rf
voltage of 1000 volts. At the Vc point of -1.25 volts, the
3-hydroxy-piperidine dimer ion of 203 m/z
[2C.sub.5H.sub.11NO+H.sup.+].sup.+ is shown, while at the Vc point
of -5.5 volts, the 102 m/z singly protonated monomer ion is shown,
as well as the 203 m/z dimer ion. The presence of a dimer ion, at
the "monomer ion Vc point" has been attributed to dimer formation
by reaction of the monomer ion with neutral analyte molecules after
the monomer ion has passed through the DMS. As a result, the dimer
ion has not been identified as a contributor to the "monomer ion Vc
point" position.
[0082] In certain configurations, dimer ions that are present at
the "monomer ion Vc point" may participate as part of an ion'
equilibrium, consisting of monomer ion and monomer-neutral molecule
cluster ions, that comprise the "monomer ion Vc point". It appears
that, in many cases, the intensity of dimer/cluster ion signal
offered by the mass spectra of a MS analyzer in conventional
systems may be significantly under representing the true
contribution of the dimer/cluster ions to the equilibrium. When
viewing a Vc point as an ion' equilibrium, the effective cross
sectional area for that point must take into account the cross
sectional area for each ion and their contributing amounts. It is
likely that conventional mass spectrometry by itself is unable to
provide an accurate view of the ion' equilibrium composition that
contributes to a particular Vc point. Specifically, there is a high
potential for de-clustering of non-covalently bound ions as they
pass through the MS inlet via the MS cone 228. Thus, in certain
embodiments, the ESI-DMS-MS system 200 employs the lowest feasible
MS cone 228 voltage setting in order to maintain as much
dimer/cluster ion signal as possible.
[0083] If the dimer ion is to be considered a true component of the
monomer ion Vc point, it is necessary to define it's presence in
two well separated Vc points for a given Rf voltage. In certain
configurations, it appears that two types of dimer formations are
present, and are responsible for the differences in Vc point
position. Equations 1 and 2 below show the two proposed dimer ion
formations. MH.sup.+.sub.(gas)+M.sub.(gas).fwdarw.MH.sup.+. . .
M.sub.(gas) Eq. 1 M.sub.(sol.)
+M.sub.(sol.)+H.sup.+.sub.(sol.).fwdarw.M.sup.. . . H.sup.+. . .
M.sub.(gas) Eq. 2 FIGS. 10A and 10B show the CAChe global minimum
energy conformation determination for the two proposed dimer
formations using the 3-hydroxy-piperidine compound as a model.
[0084] FIG. 10A is an exemplary view 1000 of a dimer ion structure
that may be present in the monomer Vc point of FIG. 9. FIG. 10A
represents a post electrospray ionization dimer formation between
an already protonated analyte ion and neutral analyte molecule.
This type of formation would correlate to the dimer ion that exists
as part of the monomer Vc point and is representative of a
clustering/de-clustering process. Under certain conditions, an
analyte ion could cluster with a neutral polar water molecule(s) in
the drift gas during the low field portion of the electrical
waveform, effectively increasing the cross sectional area of the
analyte ion. During the high field portion of the waveform, the
cluster would be dissociated, reducing its effective cross
sectional area. In this scenario, the dimer ion would be
continuously forming and dissociating as part of the monomer ion'
equilibrium. The stronger the attraction between the analyte ion
and analyte neutral, the longer the dimer ion exists and
contributes to the overall cross sectional area of the ion'
equilibrium.
[0085] FIG. 10B is an exemplary view 1002 of a dimer ion structure
that is formed as a shared proton between two neutral analyte
molecules which may be present in the dimer Vc point of FIG. 9.
This type of dimer would form during the electrospray process in
which two neutral analyte molecules compete for the proton
addition, resulting in a stable shared proton dimer configuration.
The shared proton dimer model has previously been demonstrated by
infrared multi-photon photo-dissociation (IRMPD) spectroscopy for
shared proton H.sub.2O dimer ions in the gas phase formed by
atmospheric ion spray. This type of dimer structure correlates to
the dimer ion present in the "dimer ion Vc point", and would
represent a structure that may not be as easily dissociated as the
dimer structure in FIG. 10A, leading to a larger overall cross
sectional area and lower Vc point position. This type of dimer
structure may also be favorable for higher degrees of clustering
which will be discussed later herein.
Monomer Ion' Equilibrium-Core Mechanism
[0086] In one exemplary configuration, three drift gas modifiers
were selected to investigate trends with regards to electrostatic
and H-bonding based interactions between the neutral gas phase
modifier molecules and analyte ions. Preliminary tests with
hydrocarbon gas phase modifiers, acting as non-polar controls,
indicated that having electrostatic attraction between the modifier
and analyte ions was vital to altering the analyte ion's
differential mobility behavior. It was expected that an
electrostatic attraction would exist between the hydroxy group of
the alcohol modifiers and the positively charged N on the
protonated monomer analyte ions. Equation 3 below depicts the
adduct ion formed between a protonated analyte monomer ion and
neutral modifier molecule.
MH.sup.+.sub.(gas)+Modifier.sub.(gas).fwdarw.MH.sup.+. . .
Modifier.sub.(gas) Eq. 3
[0087] FIG. 11 is an exemplary view 1100 that shows the CAChe
global minimum energy conformation determination for the
interaction between the protonated piperidine analyte and the
neutral 2-propanol molecule. As anticipated, the hydroxy group in
2-propanol coordinates with the positively charged N in
piperidine.
[0088] FIG. 12 is a combined graphic view 1200 of the DMS spectra
1204 for piperidine at an Rf=1000 volts with no modifier and
associated mass spectrometric spectra 1202 and the DMS spectra 1208
with a 2-propanol modifier and associated mass spectrometric
spectra 1206. FIG. 12 compares the DMS spectra for piperidine with
and without the 2-propanol drift gas modifier at an Rf voltage of
1000. The mass spectra 1202 and 1206 were collected at the Rf and
Vc settings corresponding to the monomer ion peak maximum for each
DMS spectra 1204 and 1208. The m/z 86 ion corresponds to the
protonated piperidine monomer ion C.sub.5H.sub.11NH.sup.+, the m/z
171 ion is the protonated piperidine dimer ion
[C.sub.5H.sub.11N+C.sub.5H.sub.11NH.sup.+].sup.+, and the m/z 146
ion is the 2-propanol-piperidine adduct ion
[C.sub.5H.sub.11NH.sup.++C.sub.3H.sub.7OH].sup.+. It can be seen
that the addition of the 2-propanol modifier produced a new
analyte-modifier adduct ion (the m/z 146 ion), which resulted in a
shift of the monomer Vc point to a lower Vc, reflecting an increase
in the overall cross sectional area of the monomer ion' equilibrium
in the presence of the 2-propanol drift gas modifier. While the MS
analyzer 240 ion signal intensity of the non-covalently bound ions
is likely not quantitative with respect to the monomer ion signal,
the dimer and modifier adduct ion signal intensities relative to
the monomer ion signal do provide valuable comparative data.
[0089] The mass spectra 1202 and 1206 indicate that the addition of
the 2-propanol modifier to the drift gas induces a competitive
formation in favor of the analyte-alcohol adduct ion over that of
the dimer ion. Table 1 shows the calculated minimum conformation
energy values and surface volume values for the piperidine dimer
and piperidine-2-propanol adduct ion complexes. TABLE-US-00001
TABLE 1 Minimum conformation energy and surface volume values for
piperidine dimer and 2-propanol adduct complexes Minimum
Conformation Surface Energy (kcal/mol) Volume
piperidine(+)-piperidine 6.3 7075 piperidine(+)-2-propanol -16.1
6535
[0090] The lower minimum energy value for the adduct ion indicates
a greater likelihood of formation for the adduct ion than the dimer
ion, consistent with the mass spectral data. The CAChe surface
volumes shown in Table 1 provide values reflective of the cross
sectional area for the complex ions'. While the dimer ion has a
greater cross sectional area than the modifier adduct ion, a larger
contribution of modifier adduct ions to the monomer ion'
equilibrium, compared to dimer ion contribution when no modifier is
used, could result in an overall increase in the monomer ion'
equilibrium's cross sectional area when the 2-propanol modifier is
used.
[0091] This explains the shift of the monomer ion Vc point to a
lower Vc with the addition of the 2-propanol modifier to the drift
gas. This type of effect is referred to as the Core interaction
mechanism, where the drift gas modifier molecule interacts with the
analyte ion to form a new ion entity that exists as part of the
analyte ion' equilibrium. The same Core mechanism can be seen in
FIG. 13 for the dimethyl-piperidine analyte with and without
2-butanol as a drift gas modifier.
[0092] FIG. 13 is a combined graphic view 1300 of the DMS spectra
1304 for dymethyl-piperidine at an Rf=1000 volts with no modifier
and associated mass spectrometric spectra 1302 and the DMS spectra
1308 with a 2-butanol modifier and associated mass spectrometric
spectra 1306. The DMS spectra 1304 shows the m/z 114
dimethyl-piperidine monomer ion C.sub.7H.sub.15NH.sup.+. The m/z
227 dimer ion [C.sub.7H.sub.15N+C.sub.7H.sub.15NH.sup.+].sup.+ is
present at an ion signal only slightly above the noise as shown in
the mass spectra 1302. The DMS spectra 1308 shows the monomer ion
plus the presence of the dimethyl-piperidine-2-butanol adduct ion
[C.sub.7H.sub.15NH.sup.++C.sub.4H.sub.9OH].sup.+ at m/z 188. FIGS.
9(a) and 9(b) show the shift of the dimethyl-piperidine monomer Vc
point from -8.6 to -6.2 Vc with the addition of 2-butanol vapors to
the drift gas.
[0093] The 3-hydroxy-piperidine analyte was selected as a test
compound because of its increased potential for hydrogen bonding
and electrostatic interactions due to the presence of the hydroxyl
group. As previously shown in FIG. 8, the 3-hydroxy-piperidine
analyte readily forms the greatest dimer/monomer ion ratio out of
all five test compounds.
[0094] FIG. 14 is a combined graphic view 1400 of 3
hydroxy-piperdine at an Rf=1000 volts showing the DMS spectra 1404
with no modifier, the DMS spectra 1408 with a 2-propanol modifier,
and the DMS spectra 1412 with a 2-butanol modifier along with their
associated mass spectrometric spectra 1402, 1406, and 1410
respectively. The DMS spectra 1404, 1408, and 1412 show the mass
spectra collected for the monomer Vc point from each condition. The
DMS spectra 1404 shows the m/z 102 3-hydroxy-piperidine monomer ion
as well as the m/z 203 singly charged dimer ion
[C.sub.5H.sub.11NO+C.sub.5H.sub.11NOH.sup.+].sup.+. The DMS spectra
1408 and 1412 illustrate the presence of the m/z 162 and m/z 176
adduct ions corresponding to the 2-propanol adduct
[C.sub.5H.sub.11NOH.sup.++C.sub.3H.sub.7OH].sup.+ and 2-butanol
adduct [C.sub.5H.sub.11NOH.sup.++C.sub.4H.sub.9OH].sup.+ ions
respectively. In both of the DMS spectra 1408 and 1412, the
3-hydroxy-piperidine dimer ion is still present at a significant
signal relative to the monomer ion. This is different than
demonstrated for the piperidine and dimethyl piperidine compounds,
where no dimer ion signal was present when the drift gas modifiers
were used. Table 2 shows the calculated minimum conformation energy
values and surface volume values for the 3-hydroxy-piperidine
dimer, 3-hydroxy-piperidine-2-propanol adduct, and
3-hydroxy-piperidine-2-butanol adduct ion complexes. TABLE-US-00002
TABLE 2 Minimum conformation energy and surface volume values for
3-hydroxy-piperidine dimer, 2-propanol adduct, and 2-butanol adduct
complexes Minimum Conformation Energy Surface (kcal/mol) Volume
3hydroxy-piperidine(+)-3hydroxy-piperidine -29.7 7348
3hydroxy-piperdine(+)-2-propanol -31.9 6529
3hydroxy-piperidine(+)-2-butanol -30.2 7178
[0095] The minimum conformation energy values for the dimer and
monomer-alcohol adduct complexes are very similar, all within 2.2
kcal/mole difference of each other. This differs significantly from
the 22.4 kcal/mole difference between the piperidine dimer ion and
piperidine-2-propanol adduct ion energy values shown in Table 1.
Unlike what has been demonstrated with the piperidine and dimethyl
piperidine compounds, the 3-hydroxy-piperidine compound provides
similar clustering strength to form gas phase dimer ions as it does
monomer-alcohol adduct ions, resulting in a more balanced
contribution to the overall monomer ion' equilibrium from both
dimer (larger surface volume) and adduct ions (smaller surface
volumes). This results in an overall decrease in the effective
cross sectional area for the monomer ion' equilibrium when the
2-propanol or 2-butanol drift gas modifiers are used compared to no
modifier. This decrease in the effective cross sectional area of
the monomer ion' equilibrium is reflected in the monomer ion Vc
point shift to a larger Vc value with the use of either the
2-propanol or 2-butanol drift gas modifier, as shown in FIG.
14.
Monomer Ion' Equilibrium-Facade Mechanism
[0096] Understanding and modeling the nature of the gas phase
interactions that govern an analyte ions differential mobility
behavior is critical to quantization of certain ions. It is clear
that each analyte is unique, even within the same class of
compounds, and that a given drift gas modification may have
opposite effects on shifting analyte Vc point positions for
different analytes. However, the Core mechanism does not appear to
be the only mechanism by which drift gas modifications can have
significant effects on an analyte ion's differential mobility
behavior.
[0097] The pentamethyl-piperidine compound was selected as a test
analyte for the steric hindrance it provides with regard to spatial
access to it's protonated N. Of all five piperidine related test
compounds, pentamethyl-piperidine has the most sterically blocked
N, and demonstrated zero dimer ion signal as shown in FIG. 8.
[0098] FIG. 15 is a combined graphic view 1500 of
pentamethyl-piperdine at an Rf=1000 volts showing the DMS spectra
1502 with no modifier, the DMS spectra 1506 with a cyclopentanol
modifier, the DMS spectra 1510 with a 2-butanol modifier, and the
DMS spectra 1514 with a 2-propanol modifier along with their
associated corresponding monomer Vc point mass spectrometric
spectra 1504, 1508, 1512, and 1516 respectively. The mass spectra
1504, 1508, 1512, and 1516 corresponding to the monomer Vc points
1502, 1506, 1510, and 1514 show the presence of only the m/z 156
pentamethyl-piperidine monomer ion, C.sub.10H.sub.21NH.sup.+, for
all four conditions. None of the mass spectra 1504, 1508, 1512, and
1516 demonstrate dimer or drift gas modifier adduct ions. Despite
no mass spectrometer signal for dimer or modifier adduct ions in
the monomer Vc points, significant monomer Vc point shifts to
larger Vc values occurred with 2-propanol and 2-butanol, as well as
a slight Vc shift with cyclopentanol. The monomer Vc point shifts
indicate a decrease in the effective cross sectional area for
monomer ion' equilibria when the drift gas modifiers are used.
Thus, in certain embodiments, a second gas phase drift gas modifier
interaction mechanism is employed in which electrostatic attraction
between the analyte ion and neutral molecule is still the governing
force.
[0099] However, unlike the proposed Core mechanism, the interaction
between the analyte ion and neutral molecule is short lived or of
reduced attraction, such that an analyte-modifier adduct ion is not
formed. The effect seen on analyte differential mobility behavior
through this mechanism is one of decreasing an analyte ion's
conformational freedom which reduces its cross sectional area. The
total conformational space or freedom of an analyte ion takes into
account all energetically possible bond angles and rotations.
Multiple short lived attractions between the analyte ion and drift
gas modifier may restrict the analyte ion's conformational
freedom.
[0100] FIG. 16A is an exemplary view 1600 of a protonated
pentamethyl-piperidine complex with and without an associated
molecular electron density cloud. FIG. 16B is an exemplary view
1602 of a neutral 2-propanol complex with and without an associated
molecular electron density cloud. FIGS. 16A and 16B demonstrate the
reduction in conformational freedom imparted on the analyte ion by
the modifier molecule. The methyl side chains of
pentamethyl-piperidine are restricted from the space occupied by
2-propanol, resulting in an overall decrease in the effective cross
sectional area for the pentamethyl piperidine ion. This effect may
be referred to as the Facade interaction mechanism.
[0101] While both the Core and Facade mechanisms have been
described as two independent gas phase interaction mechanisms, both
mechanisms likely work synergistically on a sliding scale between
the two mechanisms, dependent on the specific analyte-modifier
interaction. The data presented for the piperidine,
dimethyl-piperidine, and 3-hydroxy-piperidine analytes reflected
interactions with a strong Core mechanism component, while the
pentamethyl-piperidine data reflected interactions with a strong
Facade mechanisms component. While no cluster/dimer ions were
demonstrated in FIG. 15, the mass spectra may not provide accurate
cluster ion representation and the Core mechanism may have
contributed to the overall differential mobility behavior of
pentamethyl-piperidine with the drift gas modifiers. The sliding
scale model for the two mechanisms can be viewed in relation to the
various energetically possible conformations between a specific
analyte ion and neutral molecule interaction. In certain
circumstances, some compound interactions may favor the Core
mechanism, resulting in 90% of the possible conformations leading
to the formation of a new cluster ion (Core mechanism), while 10%
of the conformations reflect the Facade mechanism. For different
compounds the scale may slide towards the Facade mechanism. Other
compound interactions may result in only 10% of the conformations
having the Facade effect and the other 90% of conformations having
no effect at all, indicating that the conformations prevented
attraction between the two molecules sufficient for either
mechanism.
[0102] In one embodiment, Molecular modeling (MM) is employed to
investigate and/or predict the Core and Facade mechanisms for the
monomer ion equilibrium interactions. Molecular modeling data for
global minimum conformation energy and surface volume were
collected for various test conditions. Additionally, the monomer
analyte ion's change in conformational freedom was determined for
each of the five analytes.
[0103] For analyte conditions with no drift gas modifier, the MM
complex data represents the interaction between one protonated
analyte ion and one neutral analyte molecule (dimer ion). For
analyte conditions with a drift gas modifier, the MM complex data
represents the interaction between one protonated analyte ion and
one neutral modifier molecule (analyte-modifier ion). Table 3
provides the MM data, representative of the monomer ion' equilibria
complexes, for all study conditions. TABLE-US-00003 TABLE 3 Minimum
conformation energy and surface volume values for the proposed
monomer ion' equilibrium interactions for various conditions
Minimum conformation Change in energy Surface Conformal (kcal/mol)
Volume Energy piperidine(+)-piperidine 6.3 7075 19.7
piperidine(+)-2-propanol -16.1 6535 piperidine(+)-2-butanol -15.3
6739 piperidine(+)-cyclopentanol -7.2 6949
dimethylpip(+)-dimethylpip 9.1 8623 25.8 dimethylpip(+)-2-propanol
-13.7 7173 dimethylpip(+)-2-butanol -12.9 7644
dimethylpip(+)-cyclopentanol -4.9 7689 tetramethpip(+)-tetramethpip
23.1 9823 25.2 tetramethpip(+)-2-propanol -6.7 7811
tetramethpip(+)-2-butanol -1.5 8270 tetramethpip(+)-cyclopentanol
2.53 8343 pentamethpip(+)-pentamethpip 45 10531 25.1
pentamethpip(+)-2-propanol 10.7 8070 pentamethpip(+)-2-butanol 12.3
8565 pentamethpip(+)-cyclopentanol 20.2 8576
3hydroxypip(+)-3hydroxypip -29.7 7348 24.5
3hydroxypip(+)-2-propanol -31.9 6529 3hydroxypip(+)-2-butanol -30.2
7178 3hydroxypip(+)-cyclopentanol -22.5 7050
[0104] Notable is the correlation between the dimer ion minimum
conformation energy values in Table 3 and the actual MS dimer ion
signal intensity percentages shown in FIG. 8. In order to
investigate the effect that a Facade interaction may have on
changing an analyte ion's differential mobility behavior, the
potential for restriction of the monomer ion's conformational
freedom has to be determined. This is represented by calculating
the change in surface volume between the global minimum
conformation of the monomer ion alone and the surface volume of the
superimposed eight lowest energy conformations of the monomer
ion.
[0105] FIG. 17 is an exemplary view 1700 that shows the minimum
conformation of the tetramethyl-piperidine monomer ion as well as
the superimposed eight lowest energy conformations of the
tetramethyl-piperidine monomer ion. The calculated percent change
in surface volume for all five analyte monomer ions is reported in
Table 3 in terms of the change in conformational freedom (displayed
in the dimer ion row for each analyte).
[0106] As described previously, the DMS dispersion plots, DMS
spectra, and mass spectra for various Rf and Vc settings were used
to accurately identify the monomer and dimer ion' equilibria Vc
point positions for various Rf settings. Based on this data, Rf
versus Vc plots were generated for each test compound and drift gas
modifier condition. A second order polynomial trend line was
calculated and included in the plots for each condition.
[0107] FIG. 18 is a combined graphic view 1800 of various monomer
ion equilibrium plots of Rf versus Vc under various conditions.
FIG. 18 demonstrates the plots for various monomer ion' equilibria
generated by the ESI-DMS-MS system 200. Plot 1802 compares the
differential mobility behavior of the five test analytes when no
drift gas modifiers were used. Plot 1804 demonstrates the
differential mobility behavior for piperidine monomer ion
equilibria with and without the three different drift gas
modifiers. Plots 1806, 1808, 1810, and 1812 show the differential
mobility behavior for the four other analyte monomer ion equilibria
with and without the three drift gas modifiers. Table 3
demonstrates that piperidine, due to its lack of side chains, has
the smallest change in conformational freedom of all five analyte
monomer ions. This would indicate that the potential for Facade
mechanism interactions to decrease the effective cross sectional
area of the piperidine monomer ion' equilibrium is less than for
all of the other analytes. This potentially explains a monomer Vc
point shift to a greater Vc value for dimethyl-piperidine and
tetramethyl-piperidine with the 2-propanol modifier but not for
piperidine with the same modifier. The combined effects of the
Facade and Core mechanisms provide an overall decrease in the cross
sectional area for the monomer ion' equilibrium of
dimethyl-piperidine and tetramethyl-piperidine with the 2-propanol
modifier. However, the reduced potential for the Facade mechanism
effect in the piperidine monomer ion' equilibrium, results in a
more dominant Core mechanism effect.
Dimer Ion' Equilibrium
[0108] As discussed previously herein, two dimer ion formations
have been observed, reflecting the dimer ion presence in both a
"dimer ion Vc point" and "monomer ion Vc point". It is presumed
that the shared proton dimer structure shown in FIG. 10B
corresponds to the dimer Vc point. It is proposed that this dimer
structure is not easily dissociated in the DMS filter 210 electric
field and fosters clustering with other molecules.
[0109] FIG. 19 is a combined graphic view 1900 of the DMS spectra
1904, highlighting the dimer Vc point positions, for piperidine
dimer at an Rf=1000 volts with no modifier and associated mass
spectrometric spectra 1902 and the DMS spectra 1908, highlighting
the dimer Vc point positions, with a 2-propanol modifier and
associated mass spectrometric spectra 1906. The use of the
2-propanol drift gas modifier provides a shift in the dimer Vc
point position to a larger Vc, reflecting a decrease in the overall
cross sectional area of the dimer ion' equilibrium. The m/z 171 ion
is the shared proton piperidine dimer ion
[C.sub.5H.sub.11N+H.sup.++C.sub.5H.sub.11N].sup.+. The other ions
present in the mass spectra 1902 and 1906 are possibly dimer based
adduct ions contributing to the dimer ion' equilibrium or
independent ions with similar differential mobility behavior as the
piperidine dimer ion' equilibrium.
[0110] FIG. 20 is a combined graphic view 2000 of the DMS spectra
2004, highlighting the dimer Vc point positions, for
Dimethyl-piperidine dimer at an Rf=1000 volts with no modifier and
associated mass spectrometric spectra 2002 and the DMS spectra
2008, highlighting the dimer Vc point positions, with a 2-butanol
modifier and associated mass spectrometric spectra. The m/z 227 ion
is the shared proton dimethyl piperidine dimer ion
[C.sub.7H.sub.15N+C.sub.7H.sub.15N+H.sup.+].sup.+. As is the case
with piperidine, the addition of the drift gas modifier results in
a dimer Vc point shift to a greater Vc. For both piperdine and
dimethyl-piperidine, no dimer-modifier adduct ions are present in
the mass spectra 2002 and 2006. However, at the dimer Vc points of
both analytes, without any modifier, the presence of a dimer+62 m/z
ion and dimer+76 m/z ion are shown.
[0111] For piperidine, these ions correspond to the m/z 233 and 247
ion peaks, whereas for dimethyl-piperidine correspond to the m/z
289 and 303 ion peaks. Both of the analytes dimer Vc points contain
ions with the same mass difference, indicating that these are
adduct/cluster ions containing the dimer. For both piperidine and
dimethyl piperidine, the addition of the drift gas modifier
decreases the intensity of the cluster ions with respect to the
dimer ion, reflecting a reduction in clustering through the use of
the modifier.
[0112] In certain circumstances, the reduced clustering results in
a decrease to the dimer ion' equilibria effective cross sectional
area, resulting in a dimer Vc point shift towards a larger Vc.
Although the underlying interaction mechanism(s) taking place is
not clear, it appears that a core type mechanism may be taking
place, but the dimer-modifier adduct ions are just not visible in
the mass spectra. Alternatively, the Facade type interaction may be
dominating where the modifier disrupts the clustering and/or
reduces the conformational freedom of the dimer ion. Regardless of
the mechanism(s), the only drift gas modifier effect on the
differential mobility behavior for all analyte dimer ion'
equilibria, was a shift towards a larger Vc.
[0113] FIG. 21A is a Rf versus Vc plot 2100 of the piperidine dimer
ion equilibrium under various conditions. FIG. 21B is a Rf versus
Vc plot 2102 of the Cis-dimethylpiperidine dimer ion equilibrium
under various conditions. FIG. 21C is a Rf versus Vc plot 2104 of
the 3-Hydroy piperidine dimer ion equilibrium under various
conditions. Under certain conditions, these three analytes were
capable of generating enough dimer ion signal to detect and track
the shifts for the dimer ion' equilibria. As shown in FIG. 21C, no
dimer ion' equilibrium was present for the 3-hydroxy-piperidine
with 2-butanol condition due to lack of MS analyzer 240 dimer ion
signal.
[0114] DMS has demonstrated successful gas phase ion separation at
atmospheric pressure for various types of compounds, enabling its
use in many areas of chemical/biological analysis. However, desired
separations are not always optimal. One of the fundamental ways in
which analyte separation can be altered is by changing the pure
bulk medium (drift gas) within which the separation is taking
place, or using modified gas compositions.
[0115] In certain embodiment, the ESI-DMS-MS system 200 employs the
Core and Facade mechanisms as key factors effecting the change in
an analyte ion's differential mobility behavior through the use of
drift gas modifications, as well as for compensating the
quantization of certain ion species based on the nature of DMS
filter 210 ion separation through, for example, molecular modeling
of the ion species. The modeling may be performed, for example, by
a software application and/or algorithm within a memory and/or
database that is executed by the controller 242. In one embodiment,
the data generated through molecular modeling of the proposed
mechanisms provides support for describing the observed DMS analyte
Vc point shifts and applying these observed shifts to later sample
analysis within the ESI-DMS-MS system 200. More particularly, the
molecular modeling data may enable the ESI-DMS-MS system 200 or any
like DMS-MS system to predict how certain analyte ions would
respond to various drift gas modifications or to predict certain
equilibrium conditions between the DMS sensor and MS analyzer 240,
and, thereby, compensate for and/or interpolate a more accurate
quantity of certain ions of a sample. Such predictions may be based
on empirical data and/or experimental observations for certain ion
species. Other predictive tools may be based on one or models of
the behavior of certain ions within the ESI-DMS-MS system 200.
[0116] It will be apparent to those of ordinary skill in the art
that certain algorithms and methods involved in the present
invention may be embodied in a computer program product that
includes a computer usable and/or readable medium. For example,
such a computer usable medium may consist of a read only memory
device, such as a CD ROM disk or conventional ROM devices, or a
random access memory, such as a hard drive device or a computer
diskette, having a computer readable program code stored
thereon.
[0117] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
* * * * *